The atom probe is an instrument that is well known to those skilled in the art which can be used to analyse samples on an atomic scale. Numerous instrument configurations based on this analysis technique are described in the work entitled “Atom probe field Ion microscopy”, by Miller et al., published in 1996 by Clarendon Press/Oxford.
For such an analysis, it is conventional to use a pointed sample, that is: a sample with a pointed shape, raised to a given potential relative to the potential of the detector and to have, in the vicinity of this sample, an electrode raised to an intermediate potential between that of the sample and that of the detector.
It is also conventional to have, in addition to this electrode, another, grounded, electrode, or even a grating that is also grounded. Given that the detector is grounded, the ions separated from the sample follow a trajectory which projects them onto the detector without being influenced by any electrical field that might alter this trajectory. Almost all of the path of the ions is thus contained within a so-called “fieldless” space.
It is also known that an essential parameter for obtaining a fine and accurate measurement of the characteristics of the ions detected by an atom probe is the measurement of the flight time of the detected ions, that is to say the time taken by the ion concerned to travel through the space separating the sample from which they are separated from the detector. More specifically, the flight time is the time interval between an event triggering the separating of the ion and its impact on the detector. The triggering event can be an electrical pulse delivered to the electrode adjacent to the sample or a pulse of a laser beam directed to the sample. Inasmuch as the measurement of the flight time is essential in the instrument for identifying the m/q ratio of a detected ion, m being the mass of the ion and q its electrical charge, it is advantageous to increase the distance L between the sample and the detector in order to also increase the flight time. However, since the beam of emitted ions is naturally divergent, a counterpart to this increase in the distance L is that a large proportion of the emitted beam may then escape the detector, the detector having defined and necessarily limited dimensions. To overcome this drawback, it is known to interpose a convergent device such as an “Einzel” lens between the sample and the detector to focus the beam of ions on the detector. The “Einzel” lens is, moreover, a device that is well known in charged particle optics and its principle is not detailed here. For more information on “Einzel” lenses, reference can notably be made to volume 2 of the work entitled “Principles of electron optics”, by P. W. Hawkes and E. Kasper, published in 1989 by Academic Press.
Among the tomographic atom probes, there are in particular atom probes known in the literature by the name “3DAP” or “TriDimensional Atom Probe”, or even by the name “PoSAP” or “Position Sensitive Atom Probe”. These probes are advantageously characterized by the fact that, with such a detector, not only is the moment of impact, which measures the flight time of an ion, measured, but also the position, in a plane, of this impact on the detector. However, such a measurement is truly possible only if the position of the point of impact of a given ion is linked unambiguously to its position in the sample being analysed. This condition is reflected in the fact that two distinct ion trajectories should not culminate at the same point of impact on the detector.
However, although it is easy to simply vary the emission angle picked up by the detector with an Einzel lens, a strong focussing of the beam of ions emitted using such a lens leads to the appearance of a spherical aberration on the lens, an aberration that produces, on the outer trajectories, parasitic effects that greatly interfere with the operation of the 3D probe. In practice, because of this aberration, distinct trajectories end at the same point of impact.